1. Field of the Invention
The present invention relates to a radar apparatus and a radar signal processing method adapted to be installed on a movable object (vehicle, etc.) for detecting an object to be observed (hereinafter referred to as a “target”). More particularly, the invention relates to a technique that sends a radar beam as a transmitted signal and receives it as a received signal while changing the direction of an antenna so as to change the direction of the radar beam with respect to the antenna, observes a beat signal comprising up phases and down phases that is generated by mixing the frequency-modulated transmission and received signals with each other, and measures observation data including the distance, the velocity or the azimuth angle of the target relative to the antenna.
2. Description of the Related Art
In the past, the distance of a target to be detected in such a kind of radar apparatus is in the range of from a few meters to several hundred meters.
In addition, as an antenna (aerial) installed on a movable object, it is desirable to use a single transmission and reception antenna constructed for combined use with transmission and reception so as to reduce the size of the apparatus.
In order to satisfy the above requirement, there has been proposed a radar apparatus that measures the azimuth angle of a target by measuring the distance and velocity of the target by means of an FMCW (Frequency Modulated Continuous Wave) method and changing the direction of the single antenna of the combined transmission and reception construction (for instance, see a first patent document (Japanese patent application laid-open No. 11-118916) and a second patent document (Japanese patent application laid-open No. 2000-338222)).
In these conventional apparatuses, a continuous wave, comprising first modulation periods (up phases) in which the frequency thereof is becoming higher over time and second modulation periods (down phases) in which the frequency is becoming lower over time, is radiated from the combined transmission and reception antenna as a transmitted signal, and a reflected wave from a target in the form of an object to be measured is received by the transmission and reception antenna as a received signal.
Then, the received signal is mixed with the transmitted signal to generate a beat signal, which is observed in its up and down phases, so that frequencies (beat frequencies) corresponding to the target in the individual phases, respectively, are extracted, and the distance and velocity of the target are calculated from these two frequencies.
Moreover, by changing the direction of the transmission and reception antenna in accordance with the observation time, the direction of the radar beam (transmitted signal) radiated from the antenna is changed so as to scan an observation space, whereby the spectral amplitude values of the individual beat frequencies extracted from beat signals observed with radar beams of two different directions among radar beams of varying directions are measured respectively, and the azimuth angle of the target is calculated from the two spectral amplitude values thus obtained.
For instance, in the above-mentioned first patent document, attention is focused on the feature that the observation timings and the directions of radar beams in up and down phases are different between in one phase (e.g., up phase) and in the other phase (i.e., down phase) following the one phase. That is, on the basis of this relation, the azimuth angle of the target is calculated from the spectral amplitude value of the beat frequency extracted in the one phase and the spectral amplitude value of the beat frequency extracted in the other phase.
In the case of the first patent document, when the beat frequency in up phases and that in down phases are different from each other, the measured spectral amplitude value changes (increases or decreases) from its original or intrinsic value depending on the frequency thereof unless the frequency characteristic of the receiving circuit is constant. Therefore, it is desirable to make the frequency characteristic of the receiving circuit constant so as not to generate a large error in the azimuth angle calculated from the spectral amplitude value.
On the other hand, in the above-mentioned second patent document, focus is placed on the fact that the observation timings and the directions of radar beams in up and down phases are equal to each other in a certain pair of up and down phases, but different from each other in the following pair of up and down phases. That is, on the basis of this relation, the azimuth angle of the target is calculated from the spectral amplitude value of the beat frequency extracted in an up phase (or a down phase) of a certain direction of the radar beam and the spectral amplitude value of the beat frequency extracted in the up phase (or the down phase) of a direction of the radar beam adjacent to this direction.
Here, the radar signal processing method according to the above-mentioned second patent document will be specifically described while referring to an explanatory view of FIG. 12.
FIG. 12 shows the principle for calculating an azimuth angle from spectral amplitude values. In FIG. 12, the axis of abscissa corresponds to the direction of radar beams B(I) through B(I+3) and azimuth angles θ(I) through θ(I+2) which become detection results, and the axis of ordinate corresponds to spectral amplitude values A(I) through A(I+3) of the beat frequencies corresponding to targets. In FIG. 12, there exist a first target and a second target (e.g., two targets running side by side) having substantially the same distance and the same velocity at two azimuth angles θ(I), θ(I+2) indicated by outline arrows, respectively.
Here, reference will be made to the case where radar beams are transmitted toward the individual targets in directions B(I) through B(I+3 (see four parabolic curves) so as to acquire spectral amplitude values A(I) through A(I+3) of the beat frequencies corresponding to the respective targets.
In this case, in a radar beam in a direction B(I), the spectral amplitude value of the beat frequency corresponding to the first target becomes a value A(I) indicated by a “black diamond mark” in this figure. Also, in a radar beam in a direction B(I+1), the spectral amplitude value of the beat frequency corresponding to the first target becomes a value A(I+1) indicated by a “black square mark” in this figure. In addition, in a radar beam in a direction B(I+2), the spectral amplitude value of the beat frequency corresponding to the second target becomes a value A(I+2) indicated by a “black triangular mark” in this figure. Moreover, in a radar beam in a direction B(I+3), the spectral amplitude value of the beat frequency corresponding to the second target becomes a value A(I+3) indicated by a “black inverse triangular mark” in this figure. Here, note that the azimuth angles of the radar beams in the directions B(I), B(I+1) are calculated in the range of from the direction B(I) to the direction B(I+1). Therefore, as shown by the “black diamond mark” and the “black square mark”, the azimuth angle θ(I) of the first target is obtained as the result of detection from the spectral amplitude values A(I), A(I+1).
Similarly, the azimuth angles of the radar beams in the directions B(I+2), B(I+3) are calculated in the range of from the direction B(I+2) to the direction B(I+3). Accordingly, from the spectral amplitude values A(I+2), A(I+3) acquired at this time, the azimuth angle θ(I+2) of the second target is obtained as a detection result, as shown by a “black triangular mark” and a “black inverse triangular mark”.
However, the azimuth angles of radar beams in the two directions B(I+1), B(I+2), which do not share the same target, are calculated in the range of from the direction B(I+1) to the direction B(I+2).
At this time, from the spectral amplitude value A(I+1)(see the “black square mark”) corresponding to the first target and the spectral amplitude value A(I+2)(see the “black triangular mark”) corresponding to the second target, the spectral amplitude value A(I+1) of the radar beam in the direction B(I+1) is assumed to be a value indicated by a “□ mark” instead of the “black square mark”, and the spectral amplitude value A(I+2) of the radar beam in the direction B(I+2) is assumed to be a value indicated by a “Δ mark” instead of the “black triangular mark”. As a result, the azimuth angle θ(I+1) of a nonexistent target can be obtained, as shown by the “□ mark” and the “Δ mark”.
In the conventional radar apparatuses and the conventional radar signal processing methods as referred to above, for instance in the case of the above-mentioned first patent document, it is desirable to make constant the frequency characteristic of the receiving circuit, but it is difficult to make constant the frequency characteristic of an actual receiving circuit, as a consequence of which an error in the calculation of the azimuth angle of a target varies in accordance with the moving velocity of the target, thus giving rise to a problem that the calculation error of the azimuth angle becomes large particularly with respect to a target that is moving at high velocity.
On the other hand, in the case of the above-mentioned second patent document, there is also a problem that the azimuth angle θ(I+1) corresponding to a nonexistent target is detected in addition to the azimuth angles θ(I), θ(I+2) corresponding to the existing targets.